Ignore unnecessary_import in legacy analysis options (flutter#4129)

Author: srawlins

analysis_options_legacy.yaml

@@ -7,6 +7,9 @@ analyzer:
     - '**/*.mocks.dart' # Mockito @GenerateMocks
   errors:
     always_require_non_null_named_parameters: false # not needed with nnbd
+    # TODO(https://github.com/flutter/flutter/issues/74381):
+    # Clean up existing unnecessary imports, and remove line to ignore.
+    unnecessary_import: ignore
     unnecessary_null_comparison: false # Turned as long as nnbd mix-mode is supported.
 linter:
   rules: